Method and apparatus for controlling the thickness of a gate oxide in a semiconductor manufacturing process

ABSTRACT

A method and apparatus for controlling the growth of an oxide, such as a gate oxide, in a semiconductor device manufacturing process takes into consideration the ambient atmospheric pressure in order to reduce the variance in gate oxide thicknesses between wafer lots. The pressure in the oxide diffusion tube is maintained at a constant pressure near the ambient atmospheric pressure during the oxide diffusion process. Alternatively, the furnace time is changed from lot to lot as a function of changes in the ambient atmospheric pressure in order to maintain the gate oxide thickness at a constant value between wafer lots.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of semiconductormanufacturing processes, and more particularly, to the formation of anoxide layer during the manufacturing process.

BACKGROUND OF THE INVENTION

The formation of oxide layers are important steps in the manufacturingof semiconductor devices. In thermal oxidation, an oxide film is grownon a slice of silicon by maintaining the silicon in an elevatedtemperature in an oxidizing ambient, such as dry oxygen or water vapor.Thermally grown silicon dioxide is used to form a stable gate oxide forfield effect devices, for example.

Controlling the gate oxide thickness is an important manufacturingprocess control issue. As the gate oxide thickness is reduced to below150 Å, the growth kinetics changes from parabolic to linear with time.This is explained in Grove, The Physics and Technology of SemiconductorDevices, pages 22-33. In other words, for gate oxides, once thethickness is above 150 Å, it is a self-limiting process and thereforemakes it easier to control the gate oxide thickness and reduce thevariance between devices. However, for gate oxides of less than 150 Å,the linearity of the growth kinetics with time makes control over thegate oxide thickness much more difficult. The oxide thickness as afunction of time may be expressed as:$\frac{X_{o}}{A/2} = {\sqrt{1 + \frac{t + \tau}{{A^{2}/4}B} - 1}\quad {where}\text{:}}$$A \equiv {2{D\left( {\frac{1}{k_{s}} + \frac{1}{h}} \right)}}$${B \equiv {\left( \frac{2D\quad C^{*}}{N_{1}} \right)\quad {and}}},{\tau = {{\frac{x_{i}^{2} + {Ax}_{i}}{B}\quad {and}\quad C^{*}} = {Hp}_{G}}}$

with

H=Henry's Law constant

P_(G)=bulk gas pressure

D=Diffusivity of O₂ is Si

N₁=2.2×10²² SiO₂ molecules/cm³ in the oxide

k_(s)=chemical surface-reaction rate constant for oxidation

h=gas phase mass transfer coefficient

In a typical oxide/diffusion arrangement, a wafer carrier is positionedwithin an oxide diffusion tube, this wafer carrier holding a number ofwafers on which a gate oxide layer is to be grown. The processing of thewafers in the oxide diffusion tube, usually made of quartz, involvesproviding a supply of gas containing the oxidizing medium, such asoxygen or water vapor, so that it flows through the oxide diffusiontube. An oxidation furnace concentrically surrounding the oxidediffusion tube is used to heat the tube. The process is normallyperformed at ambient atmospheric pressure.

The thickness of the oxide layer is normally controlled through varyingeither the temperature and/or the furnace time, i.e. the amount of timethe wafers are subjected to the gas containing the oxidizing medium andthe elevated temperature. Although strict control is made of thetemperature and the flow of gas through the oxide diffision tube thevariance in the gate oxide thickness tends to be approximately tenpercent.

SUMMARY OF THE INVENTION

There is a need for a method and apparatus for growing gate oxide in amanner that will provide a more accurate control of the gate oxidethickness so that there will be less variance in the thickness of thegate oxide in the final product.

This and other needs are met by embodiments of the present inventionwhich provide a method for controlling the growth of an oxide in asemiconductor device manufacturing process. The present inventionrecognizes that the thickness of the oxide is proportional to the bulkgas pressure. Normally, the oxidation diffusion process is performed atambient atmospheric pressure. However, the standard atmospheric pressurevaries on a regular basis according to weather patterns, for examplefrom 28 mm Hg to 32 mm Hg. Hence, the pressure may easily vary byapproximately 6 or 7 percent. Accordingly, in certain embodiments of thepresent invention, the pressure of the gas within the oxide diffusiontube is maintained at a constant pressure. Unlike pressure diffisiontubes that have been used in the past to provide diffision at greatlyelevated pressures of several atmospheres in order to speed up thediffusion process, in the present invention the pressure is maintainedat approximately ambient atmospheric pressure. With the pressuremaintained at a constant value, and assuming that the temperature andgas flows are regulated as normal, the variance in the gate oxidethickness is reduced.

The earlier stated needs are also met by other embodiments of theinvention which provide a method of controlling the growth of an oxidein a semiconductor device manufacturing process, in which an ambientatmospheric pressure is determined. Rather than controlling the pressurein the oxide diffusion tube, the amount of time the wafer is subjectedto the elevated temperature is controlled as a function of thedetermined ambient atmospheric pressure and the temperature to which thewafer will be subjected. In other words, although the pressure is notmaintained constant, the furnace time will be set to account for theactual ambient atmospheric pressure.

The earlier stated needs are also met by an arrangement for controllablygrowing an oxide layer on a wafer in a semiconductor devicemanufacturing process. This arrangement includes an oxide diffusion tubeand a gas supply arrangement that maintains the constant gas pressurewithin the oxide diffusion tube during the growing of the oxide layer onthe wafer. This constant gas pressure is approximately ambientatmospheric pressure. The use of a gas pressure that is approximatelyambient atmospheric pressure, rather than a high pressure system, avoidsthe added danger and expense involved with such systems. The presentinvention can therefore be used with conventional oxide diffusion tubesystems if provided with a gas supply arrangement that maintains aconstant gas pressure as provided in the present invention.

In other embodiments of the invention, an ambient pressure monitor isused to determine the ambient atmospheric pressure. A controlarrangement is coupled to the ambient pressure monitor and to the heaterthat heats the oxide diffusion tube. The control arrangement controlsthe heater to heat the oxide diffusion tube for an amount of time thatis a function of the determined ambient atmospheric pressure. Thissetting of the furnace time may be done manually, automatically, or evendynamically, in response to a changing ambient atmospheric pressure,which tends not to change very rapidly.

By reducing the variance in the thickness of the gate oxide, speedvariances in microprocessors forming the final product will be reduced.Furthermore, a better control will be achieved for tunnel oxides onflash memories and electrically erasable memory cells.

The foregoing and other features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor device.

FIG. 2 is a sectional schematic depiction of an arrangement for growingoxide on a semiconductor wafer in accordance with an embodiment of thepresent invention.

FIG. 3 is a sectional schematic depiction of an arrangement for growingoxide on a semiconductor wafer for another embodiment of the presentinvention.

FIG. 4 is a sectional schematic depiction of an arrangement for growingoxide on a semiconductor wafer for still another embodiment of thepresent invention.

FIG. 5 is a sectional schematic depiction of a rapid thermal oxidationarrangement for another embodiment of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

FIG. 1 is a cross-sectional view of an exemplary semiconductor device 10to depict gate oxide. The device 10 includes a silicon substrate 12 thathas a source region 14 and a drain region 16. Field oxide 18 is providedand a gate 20 is located over gate oxide 22. The thickness of the gateoxide 22 is represented as x. The present invention improves upon thevariance in the gate oxide thickness x that is produced between wafersand wafer lots in the manufacturing process. Reducing the varianceallows an improvement in the gate oxide control, and in turn, permitstighter distributions that depend on the gate oxide control. Thisincludes microprocessor speed and writelerase cycles on programmablecells. It also improves the gate oxide reliability control.

It is to be understood that the present invention will be described withthe example of thermal oxidation to create a gate oxide. However, thepresent invention is also applicable to formation of other types ofoxide layers in addition to gate oxide layers. It is also applicable toother thermal oxides such as silicon oxynitride, thermal oxides, orthermal nitridation.

The present invention recognizes that a variance in the atmosphericpressure from the standard assumed ambient atmospheric pressure of 29.92mmHg will vary the thickness x of the gate oxide 22. The presentinvention either maintains a constant gas pressure in an oxide diffusiontube, or varies the furnace time as a function of the actual measuredambient atmospheric pressure. By using either of these embodiments tocontrol the oxide diffusion process, the gate oxide thickness x invarious wafer lots will exhibit a variance that is much reduced than ifthe ambient atmospheric pressure was not taken into consideration.

FIG. 2 is a sectional schematic depiction of a oxide diffusionarrangement in accordance with an embodiment of the present invention.The arrangement 30 includes an oxide diffusion tube 32 that is sealed atits ends by end caps 34. In typical installations, the tube 32 is madeof quartz and is vertical, although tube 32 may also be horizontal. Agas supply 36 is coupled to the inlet of one of the end caps 34. The gassupply 36 provides the carrier gas and the reaction gas. Typically, themix of gas includes nitrogen, argon and oxygen.

An oxidation furnace 38 surrounds the oxide diffusion tube 32. A liner42 separates the oxide diffusion tube 32 from a heater core 40. A timer44 controls the amount of time that the heater core will be heated(e.g., the furnace time).

Wafers 48 are loaded into the oxide diffusion tube on a wafer carrier46. The furnace time for heating the heater core 40 in the oxidationfurnace 38 is controlled by a timer 44. For a given gate oxidethickness, the timer will be set to a specific time, and this time willbe slightly varied according to the measured atmospheric pressure.

Although the setting of the timer to the same time for two differentlots of wafers should theoretically produce gate oxides having the samethickness, variations in the ambient atmospheric pressure will cause thethickness to vary from lot to lot. In order to overcome this problem,the embodiment of the present invention in FIG. 2 provides a vacuum pump50 at the exhaust of the oxide diffusion tube 32. The vacuum pump 50creates in the oxide diffusion tube 32 a slight negative pressure withrespect to ambient atmospheric pressure. As one skilled in the art wouldrecognize, to ensure that the pressure in the diffusion tube 32 remainsat a constant sub-atmospheric pressure, it must be regulated to be belowthe lowest expected atmospheric pressure, since if the atmosphericpressure drops below the pressure in the diffusion tube 32, the pressurein the diffusion tube will correspondingly drop. Therefore, the lowestexpected atmospheric pressure is determined as a function of thealtitude of the oxide diffusion tube and a lowest expected barometricpressure at that altitude. For example, the pressure created by thevacuum pump 50 may be set to a value in a range from approximately onehalf atmospheres below ambient atmospheric pressure to just slightlybelow atmospheric pressure. For example, if ambient atmospheric pressureis 29.92 mm Hg, the pressure in the oxide diffusion tube 32 may beregulated to be 26 mm Hg. The vacuum pump 50 has a regulating element tomaintain the pressure at a constant pressure in certain embodiments. Aswill be apparent to those of ordinary skill in the art, the regulatormay instead be a separate component from the vacuum pump 50.

The arrangement 30 of the present invention operates at near ambientatmospheric pressure, and therefore avoids the dangers and added expenseinherent in pressure diffusion arrangements, in which the oxidediffusion tube is pressurized to several atmospheres. The disadvantageof the pressurized diffusion tubes has made oxide diffusion at ambientatmospheric pressure the standard in the semiconductor industry.

FIG. 3 depicts another embodiment of the present invention in which thegas in the oxide diffusion tube is pressurized to slightly above ambientatmospheric pressure, (such as 32 mm Hg) up to, for example,approximately one half atmosphere above ambient atmospheric pressure.For this purpose, a pressure pump 52 (with an appropriate regulator) isemployed to maintain the constant gas pressure within the oxidediffusion tube 32 for the duration of the oxide diffusion process. Inthe embodiments of FIGS. 2 and 3, the vacuum pump 50 and the pressurepump 52 can be located at either end of the diffusion tube as there isnot a significant pressure drop across the tube 32.

With both of the embodiments of FIGS. 2 and 3, the furnace time betweenwafer lots as controlled by the timer 44 will remain the same, and thepressure in the oxide diffusion tube 32 will also be maintained at aconstant value according to the present invention. Since both thetemperature and the pressure are maintained constant from lot to lot, inboth of the embodiments of FIGS. 2 and 3, the variance in the gate oxidethickness will be reduced in comparison to the prior art methods whichoperate at ambient atmospheric pressure but do not take into account thevariations in the ambient atmospheric pressure.

FIG. 4 depicts an additional embodiment of the present invention inwhich the pressure in the oxide diffusion tube is not regulated.However, in this embodiment, the ambient atmospheric pressure is takeninto consideration in the oxide diffusion process to adjust other oxidediffusion control parameters.

A controller 54 receives signals from an ambient pressure sensor 56 thatdetects the actual ambient atmospheric pressure in the area of the oxidediffusion tube 32. Based on the actual value of the ambient atmosphericpressure, the controller 54 will set the timer 44 to control the furnacetime in order to control the gate oxide thickness to be the same fromlot to lot. For example, assume that for lot 1 the ambient atmosphericpressure, as determined by the ambient pressure sensor 56, is at 29.92mm Hg. The timer will be set at a specific value t₁ to achieve a desiredgate oxide thickness x. Now assume for lot 2 that the ambient pressurehas increased to 31 mm Hg. Since the gate oxide thickness isproportional to the bulk gas pressure, the gate oxide thickness x willbe achieved in a shorter time period than for lot 1. Accordingly, timer44 is set to a shorter time t₂ for lot 2 than it was for lot 1 in orderto achieve the same gate oxide thickness x in the wafers of lot 2 as wasachieved for the wafers of lot 1. Conversely, if the ambient pressure isless than 29.92 mm Hg when a third lot of wafers is to be processed inthe oxide diffusion tube 32, the controller 54 will set the timer 44 toincrease the amount of time t₃ of the oxide diffusion process (i.e., thefurnace time) to maintain a gate oxide thickness x.

The embodiment of FIG. 4 depicts an automatic control of the timer 44through controller 54 and an ambient pressure sensor 56 that provides asignal to the controller 54. The timer 44 may be set by the controller54 at the beginning of the oxide diffusion process so that the amount oftime that the wafer is subjected to the temperature will remain the samethroughout the oxide diffusion process. Alternatively, the time can becontrolled dynamically in a feedback control method. Thus, if theambient pressure changes during the oxide diffusion process, thecontroller 54 may lengthen or shorten the amount of time (through thetimer 44) of heating by the oxidation furnace 38. Alternatively, thetimer 44 may be manually set by an operator who has measured the ambientatmospheric pressure with a barometer and set the timer 44 accordinglyto account for the actual value of the ambient atmospheric pressure.This method therefore bypasses the controller 54 and the ambientpressure sensor 56, but does not provide dynamic feedback for changingambient pressure conditions.

Although the present invention has been described thus far with theexample of thermal oxidation/diffusion tubes, the above-describedtechniques are also applicable to RTO (rapid thermal oxidation)equipment and techniques. An exemplary embodiment of an RTO processarrangement is depicted in FIG. 5. An RTO chamber 60 receives a wafer 62(or a plurality of wafers 62) that is to be processed. Supply gas isprovided to the chamber 60 from a gas supply 64 through a valve 66 thatis under the control of a controller 68. An ambient sensor 70 comparesthe atmospheric pressure to the pressure of the chamber atmosphere andprovides a comparison signal to the controller 68. A timer 72 providesthe furnace time for the wafers 62 in the chamber 60 to the controller68. The chamber 60 is heated by, for example, infrared lamps 74. Thetemperature in the chamber 60 is determined from a value provided by athermocouple 76 to the controller 68.

The control of the RTO process equipment depicted in FIG. 5 is the sameas in the embodiment of the invention depicted in FIGS. 2-4.

The present invention reduces the variance in the thickness of oxidelayers, such as gate oxide, by taking into consideration the actualambient atmospheric pressure. The present invention allows existingoxide diffusion arrangements at ambient atmospheric pressures to beretrofitted at low expense to control the pressure in the oxidediffusion tube during the oxide diffusion process. Alternatively, theamount of time of the oxide diffusion process is changed in certainembodiments to account for differences in the ambient atmosphericpressure from the standard, assumed atmospheric pressure. Again, thisreduces the variation in the gate oxide thickness from lot to lot.

Although the present invention has been described and illustrated indetail, it is to be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

What is claimed is:
 1. A method of controlling growth of an oxide in asemiconductor device manufacturing process, comprising the steps of:positioning at least one object on which oxide is to be grown within anoxide diffusion tube; maintaining a constant pressure of gas within theoxide diffusion tube, the pressure being approximately ambientatmospheric pressure, regardless of changes to ambient pressure outsidethe oxide diffusion tube; and growing the oxide on the object until athickness of the oxide is achieved; wherein the step of maintaining aconstant pressure includes determining a lowest atmospheric pressure asa function of the altitude of the oxide diffusion tube and a lowestexpected barometric pressure at that altitude, and producing a negativepressure relative to atmospheric pressure.
 2. The method of claim 1,wherein the pressure of gas within the oxide diffusion tube ismaintained at a constant pressure that is within a range betweenapproximately ½ atmospheres below the ambient atmospheric pressure andapproximately ½ atmospheres above the ambient atmospheric pressure. 3.The method of claim 2, wherein the step of growing the oxide includesregulating temperature in the oxide diffusion tube and an amount of timethe object is subjected to the constant pressure and the regulatedtemperature, the thickness of the oxide being a function of the constantpressure, the regulated temperature and the amount of time.
 4. Themethod of claim 1, wherein the step of producing a negative pressureincludes applying a low pressure source at an exhaust end of the oxidediffusion tube.
 5. A method of controlling growth of an oxide in asemiconductor device manufacturing process, comprising the steps of:positioning at least one object on which oxide is to be grown within anoxide diffusion tube; maintaining a constant pressure of gas within theoxide diffusion tube, the pressure being approximately ambientatmospheric pressure; and growing the oxide on the object until athickness of the oxide is achieved; wherein the step of maintaining aconstant pressure includes determining a lowest atmospheric pressure asa function of the altitude of the oxide diffusion tube and a lowestexpected barometric pressure at that altitude, and producing a negativepressure relative to atmospheric pressure.
 6. The method of claim 5,wherein the pressure of gas within the oxide diffusion tube ismaintained at a constant pressure that is within a range betweenapproximately ½ atmospheres below the ambient atmospheric pressure andapproximately ½ atmospheres above the ambient atmospheric pressure. 7.The method of claim 6, wherein the step of growing the oxide includesregulating temperature in the oxide diffusion tube and an amount of timethe object is subjected to the constant pressure and the regulatedtemperature, the thickness of the oxide being a function of the constantpressure, the regulated temperature and the amount of time.
 8. Themethod of claim 5, wherein the step of producing a negative pressureincludes applying a low pressure source at an exhaust end of the oxidediffusion tube.